Vibration

This one is about to drive me and the mechanic crazy chasing this down!

92 RS - I am getting way too much feel from the road in the form of a (for the lack of a better description) a bumping vibration that seems to transfer to the body of the car and can be felt in the floor pan and seat bottom.
This is noticeable at just about any speeds 20 mph and up and is more pronounced with the higher speeds. It's almost like there is a hard or flat spot in the tires, but there isn't. I've had a new set of Goodyear Eagles on it at first and those showed signs of being out of round and good year replaced 2 of them and the problem was still present, so i found a shop that had a road force balance machine and had the tires re mounted and balanced according to those specifications (problem still existed). So I changed out the shocks and struts and did the OEM ride height measurements to verify that there wasn't any spring sag. (problem still there) Checked harmonic balancer and replaced timing chain set, new torque converter and had the tranny completely gone thru with all new bushings and everything, replaced U Joints and checked for bent drive shaft and slung balance weights, replaced front rotors and wheel bearings (inner and outter), replaced rear drums, had the wheels checked for out of round or bent. Finally, i decided to try another brand of tires (new set of BFG's) . Problem still exists!

In short im runnig out of things to fix or replace and 10 new tires later i'm no better off trying to solve this. :banghead:

is there anything inside the rear axle / differential that can cause this?

We are running out of ideas / options on this and any input or ideas would be greatly appreciated . There has got to be some little something we are missing

Have you had an alignment? After you replace anything in your suspension it's recommended to get one.

Yes its had a 4 wheel thrust angle alignment

3rd gen rear axles are inherently cheesy for lack of better wording and bend very very easily. :2cents:

Could also be a missing tooth on the ring/pinion :dunno:

Subframe bushings?

flexplate. im not sure if your motor is externally balanced but if it is it could be the flexplate ("flywheel") my 400 sbc is externally balanced and it has weights on the flexplate

The flex plate was checked out thoroughly when i had the transmission re-worked.
The vibration is not coming from the engine or transmission - today we road tested it , and at 50 mph put it in neutral and turned the engine off.
vibration still there.

The mechanic thinks it may be an outter axle bearing but cannot be sure without tearing into it.

And the saga continues....................:disappoin

I had a mechanic check front wheel bearings (those were fine no problem), and tear into the rear end to see what he could find.
After a thorough ispection of the ring gear and pinion , carrier bearings, pinion bearings, and spider gears, no problem could be found. No broken teeth or chips . No metal shavings. No play in the bearings and minimal wear to ring gear and pinion. The differential is good and tight.
The axles were inspected and there was no scoring or flat spots on the bearing roller area and the axles are straight no bends or warpage.
Replaced the axle bearings just to be sure since he had it torn down anyway.
Checked all rubber bushings and the spring insulators to be sure all were intact and not hard or brittle. Those were all present and in good shape.

The Problem still exists.:banghead:

What next??

this is gonna take some intense thinking. I've been thru my whole suspension and driveline twice, and it sounds like you've covered everything. Only other possibility I can think of is the axle housing itself being bent. It's definately not common, but not impossible either.

Wouldn't a bent axle housing have shown up during the 4 wheel alignment?
my thought is that is should or other wise it would have made their measurements to be off and would have shown up on the computer spec sheet. ( i may be wrong just my thought on this)

i don't know the answer to that, unfortunately.

another test you can do to narrow the problem down is get the car on a lift or good jack stands, start the car up, put it into gear and slowly accelerate. This will narrow your problem down to your drivetrain. If you get no vibration then you have ruled out both engine and drivetrain.

Engine off and in neutral...guess what is still spinning as the rear wheels turn???? Give up? Drive shaft! If it is out of balance or out of phase it will vibrate and get worse with speed. Even if the weights are still present it may be out of balance. To check if it is out of phase, take it off at the rear, rotate it 180 degrees and hook it back up. Test drive it. If it still vibrates put the rear of the drive shaft back the way it was and do the same with the front. If that doesn't fix it, rotate the rear end of it 180 again and try it. If it still doesn't fix it, see if you can get it on a chassis dyno and run it to the road speeds where you get vibration and see if it is there still. If not it is in the front end, If it is still there it is definately in the drive line, drive shaft or rear end.